1. Field of the Invention
This invention relates to adaptive nulling in the antenna pattern in a circular array antenna and more particularly to modifying an antenna pattern of a circular antenna array to create nulls in the direction of interfering signal sources.
2. Description of the Prior Art
Multimode circular antennas possess the useful property that their main beam may be scanned by application of a linear phase shift across their mode inputs. Unlike linear arrays, their beam does not broaden with scan angle, but maintains its shape independent of scan angle. Electronically scanned circular antennas have enjoyed widespread application since they may be quickly scanned through the use of pin diode phase shifters.
In U.S. Pat. No. 4,425,567 which issued on Jan. 10, 1984, to C. P. Tresselt, a beam forming network 62 is described coupled through phase shifters 60 to a Butler matrix as shown in FIG. 3 of '567. The outputs of the Butler matrix 58 are coupled through respective elevation pattern beam forming networks to eight vertical columns, each having eight dipole antenna elements, each vertical column equally spaced around a conducting cylinder which forms a ground plane. The beam forming network 62 in '567 generates sum and difference antenna patterns having omnidirectional side lobes. Phase shifters 60 are controlled by the steering circuitry 64 which supplies the seven phase shifters with appropriate 6-bit words for steering the antenna pattern to each of 64 beam positions around the circular antenna array 50.
In U.S. Pat. No. 4,316,192, which issued on Feb. 16, 1982, to J. H. Acoraci, a beam forming network 62 is described for generating a sum and difference pattern. The beam forming network is shown coupled to a circular array antenna through phase shifters 60 and a Butler matrix 58. FIG. 2 of '192 shows that a difference pattern from the beam forming network of FIG. 7 may be formed in hardware by subtracting a sum pattern from an omnidirectional pattern to form a cardioid which in turn was added to a difference pattern to form a difference pattern with omnidirectional side lobes. Beam forming network 62 provides a sum and difference pattern which is steered around circular multimode antenna array 50 by steering circuit 64 which supplies control signals to phase shifters 60.
A publication by B. Sheleg entitled "A Matrix-Fed Circular Array for Continuous Scanning", Proc. IEEE, November 1968, pp. 2016-2027 describes and shows in FIG. 2 thereof a Butler matrix fed electronically scanned circular array.
In U.S. Pat. No. 4,178,581, which issued on Dec. 11, 1979, to R. E. Willey, Sr., a phased array antenna is shown having individual antenna elements coupled through phase shifters to a power distribution network. The phase shifters are first set by a beam steering control unit to form a narrow beam. By applying spoiling phases to the phase shifters of the array the narrow beam may be broadened substantially, for example, from three degrees beam width to eighty-eight degrees beam width, as shown in FIG. 7. Computation of spoiling phases to provide particular patterns for particular phased array antennas is disclosed.